1. Field of Invention
The present invention relates generally to a hybrid electric vehicle and specifically to a new method and system to control a combined fuel cell and battery pack power system to produce an efficient and cost-effective powertrain with acceptable drivability.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by an internal combustion engine (ICE) is well known. Vehicles powered by alternative energy sources are under development. For example, vehicles with battery powered electric traction motors are well known in the prior art. Other electric powered motor vehicles are also known having load-dependant current generators. An example of such a load-dependant system is a fuel cell system.
Fuel cells generate electrical power through an electrochemical reaction of a fuel and oxidant, such as hydrogen and oxygen. Water is the product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen, a product that is easily disposed. See generally, U.S. Pat. No. 5,991,670 to Mufford.
The desirability of an electric motor powered vehicle is clear. Nevertheless, there remains substantial room for development of ways to improve vehicle performance. For example, in a typical battery powered electric vehicle, the electric motor drive system (the load) draws current from the battery as needed to power the electric motor more or less in an xe2x80x9copen-loop.xe2x80x9d In this case, the battery automatically follows the load; it is xe2x80x9cload following.xe2x80x9d
A fuel cell power source, or other load-dependent current generators, presents a more complex challenge. Here, the electric motor drive system can no longer draw current in the xe2x80x9copen-loopxe2x80x9d fashion described above. Controls associated with these types of systems are known in the prior art. These controls must provide a xe2x80x9ccurrent commandxe2x80x9d to the fuel cell system (FCS) to adjust its power output, resulting in an instantaneous xe2x80x9ccurrent available.xe2x80x9d This allows the FCS to also be xe2x80x9cload following.xe2x80x9d
Problems, or undesirable effects, result when the actual current drawn by the load does not draw the amount of xe2x80x9ccurrent available.xe2x80x9d First, if the actual current drawn by the load is more than what the FCS makes available, the resulting high voltage DC bus created by the FCS will drop in an undesired manner, e.g., it may drop to lower than the FCS has anticipated, causing problems within the FCS. Second, if the actual DC bus current drawn by the load is bigger than what the FCS makes available, the vehicle battery pack can supply the additional current needed (the xe2x80x9cload levelerxe2x80x9d) to meet the current command.
There are other technical obstacles to the commercialization of fuel cell powered vehicles. Cold start remains a significant challenge. So far there has been no successful demonstration of cold start which is both fast and clean. There is a requirement for fast heating of a large thermal mass. The fuel processor contains a number of catalyst beds and there is a need for a compromise between significant mass for durability and lightweight construction for speed of response. Cold start will also require attention to control techniques. Batch control in process equipment has much to offer in this regard.
Fuel cell transient operation is also a problem in addition to cold start. The choice of processes and their implementation is fundamental to achieving the right transient performance. For passenger cars, transient operation is constrained by meeting emission criteria while delivering power to meet drivability criteria. Systems issues like control and xe2x80x9chybridizationxe2x80x9d are fundamental to meeting such criteria. For urban driving, mechanical energy will be dissipated as heat during frequent stops. Regenerative braking, coupled with a load leveling power, like a battery pack, can recover a significant amount of energy and thus increase fuel efficiency.
Thus, there is a need to develop an efficient, cost effective method and system to control a load following source (such as a fuel cell system) and load leveling source (such as a battery) combination while maintaining vehicle drivability.
Power control strategies for a combined fuel cell and battery power control system are known in the prior art. U.S. Pat. No. 5,929,595 to Lyons et al., discloses controls for an electric vehicle with an auxiliary source of electricity such as a diesel engine. The system attempts to conform operation to a conventional ICE vehicle while also factoring battery state of charge and using batteries for load leveling. While useful, this invention does not address the most efficient means of controlling the system.
Other electric powertrain control patents exist. U.S. Pat. No. 5,780,980 to Naito, discloses a controller for an electric car, but the fuel cell is small and only used to charge the battery when the battery state-of-charge (SOC) drops to certain limit. U.S. Pat. No. 5,820,172 to Brigham et al. describes using the possible fuel cell/battery combination to meet the power requirement with least fuel cost. This system does not depend primarily on use of a FCS for load following and a battery for load leveling. Further, this patent does not consider regenerative braking to recapture kinetic energy as well as battery assistance to help start-up (including heating fuel cell, providing power to a fuel pump, and providing power to traction motor). Nor, does this patent consider the battery and fuel cell service life, durability, and performance.
U.S. Pat. No. 5,898,282 to Drozdz et al. describes an efficient method of controlling a hybrid electric vehicle with a single source for energy generation (such as an ICE, fuel cell, or metal air cell) based on vehicle speeds, regenerative braking, and system voltage levels. Again, this control system does not address cold start for a fuel cell system. Battery SOC and cold start do not effect this strategy. Further, this patent does not consider battery use pattern effect on battery service life.
Unfortunately, there does not exist a hybrid electric vehicle control strategy to address regenerative braking, efficient battery charging to increase fuel economy, cold start, and load leveling that is efficient and cost-effective with acceptable drivability.
Accordingly, the present invention provides a new method and system to control a combined fuel cell and battery pack power system to produce an efficient and cost-effective powertrain with acceptable drivability and no emissions or reduced emissions.
It is a further object of the present control system and method invention to provide reduced vehicle maintenance cost by increasing the battery service life and fuel efficiency.
It is a further object of the present control system and method to provide reduced vehicle cost by reducing fuel cell engine size required by a hybrid electric vehicle.
It is a further object of the present control system and method to respond rapidly to load changes.
It is a further object of the present control system and method to provide rapid cold start.
It is a further object of the present control system and method to provide increased fuel efficiency by recovery, storage, and re-use of the vehicle kinetic energy normally dissipated as heat during braking.